Not applicable.
1. Technical Field
The present invention relates to optical communication networks, and in particular to a protection switching system and method within an OP-N sublayer and the use of OP-N links to simplify the management of protected connections in a such a network.
2. Background of the Invention
An important charateristic of communication systems such as fiber optical networks is the type of protection employed aghast failure of transmission in the working channels. In such systems, protection switching techniques are used to switch the transmission of traffic from working channels to protection channels, in case of failure of the working channels. Digital communication systems employ two main protection switching techniques that can be associated with corresponding network architectural layers. The layers of a transport network are described in ITU-T Recommendations G.805 (Generic Functional Architechture of Transport Networks) and G.803 (Architeture of transport networks based on the synchronous digital hierarchy (SDH). The best known SONET/SDH protection switching methods are line switching and path switching, associated with the line layer and the path layer, respectively. The layers and techniques will next be described in terms of SONET terminology, but persons skilled in the art will recognize the applicability of these concepts to other similar standards such as the SDH, or other non-standard transmission schemes.
The Line Layer refers to the maintenance span. A maintenance span forms a segment between two SONET devices, excluding the lower layer regenerators, used to regenerate the digital signals at regular intervals. A single SONET link, from one user site to another may consist of many such spans. The Line Layer manages the transport of entire SONET payloads, which are embedded in a sequence of STS frames, across the physical medium. Associated with the Line Layer there is a set of overhead (OH) bytes, called the Line Overhead (LOH), that provides the layer its ability to perform its functions, its ability to communicate with the layers that surround it and to provide certain protection and maintenance features. The LOH is used and created by SONET line-terminating equipment (LTE).
Line switching within SONET subnetworks works by restoring all working channels of an entire Optical Channel (OC)-n capacity as a single protection operation. The protection capacity is idle when the ring is operating normally and live user traffic is always sent on the working channel. In the event of a failure, the live traffic is switched to the protection fiber at both ends of the span. Channels within the xe2x80x9clinexe2x80x99 are switched this way, hence the name xe2x80x98line switchingxe2x80x99. Line switching makes use of the LOH to determine failure of the working channels.
The Path Layer covers end-to-end transmission, where end-to-end refers in this case to customer-to-customer transmission. One end of the Path is always where the bits in the SONET Synchronous Payload Envelope (SPE) originate and the other end of the Path is always where the bits in the SPE are terminated. These ends may not be the actual end user devices, such as a desktop workstation or server, but usually refer to some kind of premises SONET multiplexiig devices. A Path Overhead (POH) is associated with this layer and is considered to be part of the SPE and enables the use of functions that ensure overall transmission integrity.
In contrast to line switching, path switching can restore all working channels at a level below the entire OC-n capacity. That is, protection switching can be done on an STS-1 , the basic unit signal in an OC-n, or even at the VT (Virtual Tributary) level if that is what the STS payload is carrying. In order to do this, the signals are sent on both a working and a protection channel across the entire length of the path. The receiving end constantly monitors both and selects the best signal to be used. Path switching makes use of the POH to determine which one of the two channels, working or protection, is better.
With the introduction of high capacity transport systems such as an 80-Gbits/s line system that carries 1536 Virtual Channel(VC-3) signals, line switching may be undesirably inflexible, if 80 Gbits/s granularity is used, whereas path switching may be too complex, if VC-3 (STS-1) granularity is used.
In addition, with line switching, a high capacity line cannot support multiple independent protected subnetworks or unprotected connections. Line switching allows for complex protection schemes such as m:n protection (i.e. m protection lines for n working lines) cable route diversity, physical site diversity, equipment protection only, but at the price of adding a physical line in each case, even though most of the capacity of each line may remain unused.
Path switching allows lonely 1:1 protection since a protection channel is simultaneously used for each working channel. Therefore the protection channel may not be shared, therefore m:n protection schemes with m less than n are not possible.
Therefore there is a need in industry for a protection switching technique that overcomes, at least in part, disadvantages associated with the conventional protection switching techniques.
Open Connections
Co-assigned U.S. application Ser. No. 09/539,707 filed on Mar. 31, 2000, now U.S. Pat. No. 6,735,170 and entitled METHOD AND SYSTEM FOR ESTABLISHING CONTENT-FLEXIBLE CONNECTIONS describes a method for establishing an open connection (OP-N), mapped across a communication network. The OP-N connection includes an overhead channel to support functions such as protection switch signalling, performance monitoring and OP-N trace.
An OP-N connection is basically a bandwidth pipe with the capacity of n STS-1 SPE""s in the SONET language, or n/3 STM-1s in the SDH language, routed once from one network node to another network node. Network resources are allocated to the OP-N connection in such a way as to allow the transport of any signal combination which satisfies predetermined concatenation constraints (which are selected for maximum flexibility) and which has an aggregate capacity less than or equal to the maximum allowable capacity associated with the open connection. As a result, the traffic mix (that is, the mixture of concatenated and non-concatenated signals) conveyed through the OP-N can be changed at the end user""s convenience, without the network operator performing any action. For example, an OP-60 connection would support five STS-12c connections one day, one OC-48c and 12 STS-1 connections the next day, and two STS-24 connections and one STM-4 connection on the third day. An OP-n connection can be viewed as a virtual transport line, physically supported within a physical link, but having a capacity smaller than or equal to the capacity of the physical link. Therefore a physical high capacity network may support several OP-n subnetworks. Also, OP-N connections are capable of transiting intermediate physical nodes, along the path of signals they may carry. Protected or unprotected OP-N connections may also be carried across such a system.
Within the SONET architecture, the OP-N structure establishes a new layer between the line and the path layers. An OP-N connection is generally longer than one line, but shorter than a path. One path connection may span several OP-N connections. One OP-N connection may span several lines.
OP-N connections can be nested, i.e. an OP-m may be carried within an OP-N provided that Nxe2x89xa7m. This allows, for example, an OP-m to be established over a concatenation of protected OP-N connections. This significantly simplifies the provisioning of connections in large complex networks.
The present invention seeks to overcome the disadvantages of the prior art associated win protection switching techniques for digital communication systems.
In accordance with one aspect of the invention, there is provided an OP-N protected subnetwork of a communication system. The OP-N protected subnetwork comprises at least one working OP-N connection established between first and second locations within the subnetwork, and a protection OP-N connection established between first and second locations. The working OP-N connection is for carrying traffic of signals in normal operation from the first location to the second location and it includes a logically independent signalling channel for detecting failure of the working OP-N connection. The protection OP-N connection is for carrying the traffic of signals in the event of failure of the working OP-N connection.
In accordance with a further aspect of the invention there is provided a protection switching method comprising: sending traffic normal operation between a first and a second location through a working OP-N connection established between said first and second locations, monitoring for failure of the working OP-N connection on a logically independent sigalling channel associated with the working OP-N connection, and sending traffic between the first and second locations through a protection OP-N connection established between said first and second locations, in the event of failure of the working OP-N connection.
In accordance with yet another aspect of the invention there is provided a communication system comprising a plurality of OP-N protected subnetworks, and a protected OP-mp connection established across at least two of the OP-N subnetworks, with m less than =n. Each OP-n protected subnetwork includes at least one working OP-N connection established between first and second locations (subnetwork nodes) and a protection OP-N connection established between the first and second locations. The working OP-N connection is for carrying traffic of OP-N signals in normal operation from the first location to the second location and it includes a logically independent signalling channel for detecting failure of the working OP-N connection. The protection OP-N connection is for carrying the traffic of OP-N signals in the event of failure of the working OP-N connection. The protected OP-N connection comprises a working OP-N connection and a protection OP-N connection. The working and the protection OP-N connections connect a first location within a first OP-N protected subnetwork and a second location within a second OP-N protected subnetwork.
Among the advantages presented by the protection switching method and system of the invention are the ability to provision logical protected subnetworks that awe not constrained to the topology of the physical line systems, the ability of a single high capacity line system to support multiple protected subnetworks, the ability to provide complex protection schemes such as m:n protection within the physical line system.
Other advantages, objects, and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed description of preferred embodiments in conjunction with the accompanying drawings and claims.